Field of the Invention
The present invention relates to a power semiconductor device and in particular to a power semiconductor device having an IGBT region and a diode region for reverse conduction of the IGBT region.
Description of the Background Art
There are generally various demands on power semiconductor devices, such as loss reduction, the capacity to withstand a high voltage, and the guarantee of the safety operating area in order to prevent breakdown of elements during operation. While satisfying these demands, the decrease in size and weight of devices has also been progressing. There is also demand for the ability to fulfill the above demands at a cost as low as possible. The progress of this power semiconductor device technology leads to consideration to global environment in terms of a reduction in energy consumption.
In particular, with power semiconductor devices having functions of an insulated gate bipolar transistor (IGBT) and a free-wheeling diode (FWD), a reverse-conducting IGBT (RC-IGBT) having a structure in which both the IGBT and the FWD are formed on a single semiconductor substrate has been proposed as a method for reducing size and weight. An ordinary IGBT has only a p+ collector layer on the back surface of the semiconductor substrate, but a feature of the RC-IGBT is that both a p+ collector layer and an n+ cathode layer are formed on the semiconductor substrate. The current path of the RC-IGBT passes through the p+ collector layer when the RC-IGBT operates as an IGBT, whereas it passes through the n+ cathode layer when the RC-IGBT operates as an FWD. To suppress the switching loss of the RC-IGBT, the FWD is required to combine a low recovery current and a small forward voltage drop.
Lifetime control technology is known as a method for reducing the recovery current of the FWD, but this method has a tradeoff relationship with a forward voltage drop. In particular, in the case of the RC-IGBT, which has a FWD and an IGBT on the same substrate, lifetime control can result in an increase in ON-state voltage when the RC-IGBT operates as an IGBT.
Another problem specific to the RC-IGBT is an increase in steady-state loss caused by an increase in forward voltage due to snapback when the RC-IGBT operates as an IGBT or as a FWD, the details of which will be described below.
To maintain a low forward voltage when the RC-IGBT operates as an IGBT, it is necessary to cause conductivity modulation by forward biasing the pn junction between the p+ collector layer and an n layer including an n− drift layer so that holes are injected from the p+ collector layer into the n− drift layer. The RC-IGBT, however, has not only the p+ collector layer but also the n+ cathode layer on the back surface of the semiconductor substrate, so that there is a path through which electron current from the emitter electrode flows into the n+ cathode layer. Thus, snapback occurs until the above pn junction is turned on, and this keeps conductivity modulation from occurring. This increases the ON-state voltage of the IGBT in a low current region and accordingly increases the steady-state loss.
The same kind of problem also arises when the RC-IGBT operates as an FWD in which the reverse current flows. When a channel of an n-channel MOSFET structure that is included in the IGBT is set in an accumulation state by taking the gate voltage as positive, there is a path through which electron current from the n+ cathode layer flows toward the channel. Thus, similar snapback occurs as described above. This increases the forward voltage drop (Vf) of the diode and accordingly increases the steady-state loss.
To suppress the steady-state loss, it is necessary to suppress the aforementioned snapback. For example, Japanese Patent Application Laid-Open No. 2008-53648 discloses an RC-IGBT having a first region where an IGBT is formed, and a second region where a diode is formed. The IGBT region includes a plurality of cells. This RC-IGBT has a smaller number of current paths that can cause snapback than in the case where each cell includes both an IGBT and a diode, thus reducing the influence of snapback.
The above-described technique of Japanese Patent Application Laid-Open No. 2008-53648 can suppress snapback, but the switching loss cannot be reduced sufficiently because of a large recovery current flowing when the RC-IGBT operates as an FWD.